Last month my car started to display a “Check Spare Tire Pressure” warming when the car is turned on. It goes on for several seconds then disappears and the yellow warming icon that would usually stay if there is any issue also goes away. Now here is the thing, my car doesn’t have nor did it ever have a spare tire that has a tire pressure sensor in it. lol Anyone know why this is now happening and or how to stop it from happening every time I start the car. Thanks!!

Check the spare tire pressure. There is a sensor that reads all tire pressures and will notify you if one or more tires is low.

This happened to me a few months ago, and the dealership had to end up resetting the TPMS system, I believe. I can check my service records when I get home. They suspected it was because I recently had aftermarket wheels put on (with sensors), but they werent sure, and said this is not a common issue that they see.

Not sure if that helps at all, but thats what my situation was.

I still say check the tire pressure on the spare. That space saver spare requires 60 PSI where as the other tires are 32 PSI. It only takes a drop of one or two PSI to cause the system to tell you to check the pressure on the spare.

There is a battery in each of the tire pressure sensors, so if the PSI is 60, take the spare to a tire dealer. They will have to take the tire off the rim to replace the sensor.

I also had the same issue and dealer had to reset it. They said that the system thought I had a full size spare tire and was looking for the reading off the TPMS sensor.
If you don't have a full size spare in your car with a TPMS sensor in it, you will need to have it reset.
If the valve stem is the normal rubber one and doesn't look like your other four you don't have a TPMS in it.

Do you know if a full-size spare would fit, or would it prop up the trunk floor covering?

It would definitely prop up the flooring. I put my jump leads (about 1" thick coiled up) on top of the compact spare, and the floor was propped up.

space savers never have tpms, full size do and when replacing a module for tpms it ask if the tpms is fitted. check yes and itll show not monitored spare. check spare is an anomaly when you don't have tpms in the spare. has to be programmed out.

And is there a modified trunk panel for full size spares, or do they fit in the existing space?

No, and no.

... and on my car at least I would have to carry two different size wheels if I wanted to carry a full size spare.